1. Field of the Invention
The present invention relates to a rubber member for use in an electrophotographic apparatus (hereinafter referred to as an electrophotography rubber member) as a part such as a cleaning part, a charge-imparting part, a development part, an image-transfer part, or a paper-feed part, and more particularly to an electrophotography rubber member employed as a member such as a cleaning member for removing toner from a toner-image carrier (e.g., a photoreceptor or an image-transfer belt) which is provided with a toner image and subsequently transfers the toner image to an image-transfer member; a development member for use in a development part; or a charge-imparting member which imparts electric charge to a photoreceptor. The present invention also relates to a rubber member for separating paper sheets (hereinafter referred to as a sheet-separation rubber member), which member is to be used in a paper-feed apparatus employed in a variety of printers such as ink-jet printers and laser printers and in a variety of apparatuses for office automation (OA) such as facsimile machines.
2. Background Art
In a typical electrophotographic process, an electrophotographic photoreceptor undergoes at least the steps of charging, light-exposure, development, image-transfer, and cleaning. In such an electrophotographic process, polyurethane is employed as a member such as a cleaning member for removing toner from a toner-image carrier which is provided with a toner image and subsequently transfers the toner image to an image-transfer member; a development member for use in a development part; or a charge-imparting member which imparts electric charge to a photoreceptor.
The reason for use of polyurethane is that polyurethane exhibits excellent wear resistance and sufficient mechanical strength even though no additive such as a reinforcing agent is added. In other words, polyurethane is a material which does not stain an object to be processed.
However, polyurethane is known to have temperature-dependent physical properties. Among them, rebound resilience is particularly temperature-dependent, and the dependence poses a problem during a cleaning step carried out by use of a cleaning blade formed of polyurethane. Specifically, relatively low resilience at low temperature results in poor cleaning performance, and relatively high resilience at high temperature causes problematic chipping of an edge of the cleaning blade and generates problematic noise.
Recently, keeping pace with a trend of providing high-quality images in electrophotography, the particle size of toner has been reduced. Thus, provision of a cleaning blade of higher performance is desired.
In order to clearly illustrate the problems, a typical paper-feed part employed in a variety of OA machines will be described. As shown in FIG. 1, a conveyor part for conveying paper sheets 11 is provided. On the downstream side with respect to a conveyance direction are provided a feed roller 12 to be rotated by means of driving means and a sheet-separation rubber member 10 formed of a material such as urethane rubber, the rubber member 10 oppositely facing and being in pressure contact with the feed roller 12. In the paper-feed part, a paper sheet 11 is fed, by means of the conveyor part, into a space between the feed roller 12 and the sheet-separation rubber member 10, and is further conveyed through rotation of the feed roller 12. When a plurality of sheets 11 are fed into a space between the feed roller 12 and the sheet-separation rubber member 10, friction force generated between the sheet-separation rubber member 10 and the sheets 11 prevents simultaneous feed (double feed) of a sheet 11 which is in contact with the rubber member 10 and not in contact with the roller 12 and a sheet 11 which is in contact with the roller 12. Subsequently, when the entirety of the sheet 11 comes into contact with the roller 12, the sheet 11 is further conveyed through rotation of the feed roller 12.
However, as mentioned above, when the sheet-separation rubber member 10 is formed of conventional polyurethane rubber, anomalous noise is generated during sliding with a sheet 11.
In view of the foregoing, the present inventors have carried out extensive studies in order to solve the aforementioned problems, and have found that forming an electrophotography rubber member by use of a polyurethane produced from a polyester-polyol having an ester concentration (as defined below) of 2-8 mmol/gxe2x80x94lower than that of a conventional polyester-polyolxe2x80x94reduces temperature dependence of rebound resilience; maintains a satisfactory level of rebound resilience within a low temperature range; and suppresses increase in rebound resilience within a high temperature range, to thereby suppress damage such as chipping of an edge.
The inventors have also found that forming a sheet-separation rubber member by use of a polyurethane produced from a polyester-polyol having an ester concentration (as defined below) of 2-8 mmol/gxe2x80x94lower than that of a conventional polyester-polyolxe2x80x94reduces temperature dependence of the rebound resilience; maintains a satisfactory level of rebound resilience in a low temperature range; and suppresses increase in rebound resilience in a high temperature range, to thereby reduce noise. The present invention has been accomplished on the basis of these findings.
Thus, an object of the present invention is to provide an electrophotography rubber member which has a rebound resilience that remains satisfactorily constant with respect to change in environmental conditions and can serve as a member such as a high-performance cleaning blade. Another object of the present invention is to provide a sheet-separation rubber member which can prevent double feed of paper sheets, sheet jam, and noise and which maintains friction characteristics and exhibits excellent wear resistance.
Accordingly, in a first aspect of the present invention, there is provided an electrophotography rubber member, which is formed of a rubber elastomer comprising a polyurethane formed from a long-chain polyol containing a polyester-polyol having a number average molecular weight of 500-5,000 and an ester concentration of 2-8 mmol/g, wherein the ester concentration is defined by the following relationship:
ester concentration (mmol/g)=(amount by mol of ester groups)/(weight of the polyester-polyol). 
In a second aspect of the invention, there is provided a sheet-separation rubber member for use in a paper-feed part of an apparatus for office automation, which member is formed of a rubber elastomer comprising a polyurethane formed from a long-chain polyol containing a polyester-polyol having a number average molecular weight of 500-5,000 and an ester concentration of 2-8 mmol/g, wherein the ester concentration is defined by the following relationship:
ester concentration (mmol/g)=(amount by mol of ester groups)/(weight of the polyester-polyol). 
Preferably in the first and second aspects of the invention, the difference between the rebound resilience of the rubber elastomer at 50xc2x0 C. (RbT50) and that of the rubber elastomer at 10xc2x0 C. (RbT10) is 55% or less, the difference being obtained by the following formula:
xcex94Rb(%)=RbT50xe2x88x92RbT10. 